This invention relates generally as indicated to a high-low speed drive system for multiple spindle machines, and more particularly to certain improvements in such a drive system which permit more rapid and precise changing of the main drive shaft speed from high idle speed to low machining speed and then back to high idle speed during each complete cycle of machine operation for increased machine production.
In a multiple spindle machine, the main drive system provides a means of transmitting power and controlling the various machine tool operations, including the feeding of the bar stock, the indexing of the bar stock carrying spindles, the feeding of the tool carrying slides, and the cutting off of the finished work. All of these machine operations are controlled by a main drum shaft which is alternately driven by a main drive shaft (normally referred to as the large worm shaft) at a high idle speed for a predetermined period of time, then at a predetermined low machining speed and then back to the high idle speed during each complete cycle of machine operation.
The high speed idle time is the time required to withdraw the machine tool slides, index the spindle carrier, open each collet, feed the bar stock, close the collets, and bring the tools carried by the slides up to the work. The low speed machining time, on the other hand, is the time required to perform all machining operations. All of the motion which occurs during both the high speed idle time and low speed machining time is controlled by the main drum shaft which is driven at the desired high and low speeds by a high-low speed drive system.
Heretofore, one such known drive system included a lever operated mechanical disc clutch associated with the high speed shaft for driving the main drive shaft and thus the main drum shaft at the higher idle speed and an overrunning roll clutch associated with the low speed shaft for driving the main drive shaft and thus the main drum shaft at the lower machining speed. One drawback of such a system was that the tools could not be moved all the way up to the work during the rapid advance of the tool slides toward the work in that when the high speed mechanical disc clutch was disengaged, sufficient room had to be provided between the tools and work to permit a standard wrap around drag brake on the main drive shaft to be mechanically actuated to slow the main drive shaft down to the speed of the low speed shaft so that the overrunning roll clutch would engage to advance the tool slides under the positive control of the roll clutch before the tools engaged the work.
Another disadvantage in using such a roll clutch and drag brake was that the drag brake wore out too fast and had to be constantly adjusted because the coefficient of friction of the drag brake kept changing due to the heat generated during braking. As the brake began to wear or get out of adjustment, the tools had to be backed off even further to allow for a greater margin of error. This problem was further magnified with advancements in the index mechanisms which permitted the indexing speeds of the spindle carrier and speed of rotation of the main drive shaft to be increased during idle to reduce cycle time. Also, the use of such a standard brake had the further disadvantage that there was no convenient way to dissipate the heat generated during the braking operation.